For various undertakings such as prospecting and mining operations, national security searches for dangerous materials, and assessments of environmentally harmful substances, it is necessary to analyze solids, liquids and gases to detect the presence of valuable or hazardous elements. It is desirable to utilize a sensor device which would be capable of sensitive, real time elemental analysis to monitor or detect the presence of such elements. It is also desirable that such a sensor device be self-powered and sufficiently compact so as to be portable by an individual. In particular, a portable sensor device would facilitate elemental analysis at particular locations, thereby eliminating the need for special sample preparation and the necessity for sending samples to specially equipped laboratories. It is desired that such a sensing device be able to readily detect the presence of valuable elements such as gold, silver, palladium, platinum, and others. It is also desirable that such a device be able to readily detect the presence of hazardous elements such as trace metals including lead, mercury, arsenic, beryllium, chromium, antimony, barium, cadmium, thallium, nickel and selenium. Such a device should have high sensitivity to facilitate discovery of valuable mineral deposits. The device should also require minimal sample preparation to be effective in the field. Such a device should also be capable of detecting the presence of many different elements simultaneously.
Instrumentation and devices for the sensitive elemental analysis of materials developed to date suffer from limitations of their not being compact, portable devices, or being severely limited in terms of the range of elements and forms of matter that can be sampled. The use of plasma sources for elemental excitation or detection is currently the primary means for sensitive detection of trace elements in solids, liquids and gases. M. W. Blades et al., Application of Weakly Ionized Plasmas for Materials Sampling and Analysis, IEEE Trans. on Plasma Sci., Vol. 19, pp. 1090-1113 (1991) have reviewed such technology, which included conductively coupled plasmas, microwave-induced plasmas, and other techniques. None of the techniques so described are applicable to sensitive, real time measurements for use in a portable sensor device. Fast Fourier transform spectroscopy, as, described by J. Demirgian, Continuous Monitor for Incinerators, U.S. Department of Energy Information Exchange Meeting on the Characterization, Monitoring, and Sensor Technologies, Dallas, Tex. (Jun. 3-4, 1992) can be used for continuous, near real time monitoring of molecular gases, but is not capable of the detection of metals, especially if the metals are in particulate form. Commercial in situ detectors, such as the Bacharach Instrument Company mercury sniffer model MV-2J-W and the Pacific Northwest Laboratory Halo-sniff spectrochemical emission sensor cannot be used as portable units for real time measurements of metals in a wide range of particulate as well as vapor form.
F. C. Fehsenfeld et al., Microwave Discharge Cavities Operating at 2450 MHz, Rev. of Sci. Instrm., Vol. 36, pp. 294-298 (1965) described a number of microwave-induced plasma (MIP) resonator cavity structures. One such structure had a built in taper to reduce its height to increase the electric field strength for plasma breakdown. This device was a resonator, not a shorted waveguide. It also included a number of features that limited maximum microwave power, such as a cable connection to the source of such power. None of the devices described by Fehsenfeld are suitable for a portable sensor device.
R. M. Barnes, et al., Design Concepts for Strip-Line Microwave Spectrochemical Sources, Anal. Chem., Vol. 62, pp. 2650-2654 (1990) described a shorted strip-line microwave MIP arrangement with a dielectric tube through the device one-quarter wavelength from the shorted end. Again, the features of this device, such as the presence of the strip-line and the cable connection to the source, would limit the maximum power operation of this device. The power limit would prevent use in the high power mode which may be desirable for spectroscopic analysis of particulates and would be desirable for a portable device.
H. Matusiewicz, A Novel Microwave Plasma Cavity Assembly for Atomic Emission Spectrometry, Spectrachimica Acta, Vol. 47B, pp. 1221-1227 (1992); Y. Okamoto, Annular-Shaped Microwave-Induced Nitrogen Plasma at Atmospheric Pressure for Emission Spectrometry of Solutions, Analytical Science, Vol. 7, pp. 283-288 (1991); and D. K. Smith, et al., Microwave Atmospheric Pressure Plasma Torch, Characteristics and Application, 27th Microwave Symposium, Washington, D.C. (Aug. 2-5, 1992) described sigher power MIP devices connected to the microwave source directly by the waveguide. These devices are unsuitable for use in portable devices.
Other microwave-induced plasma-atomic emission spectroscopy devices are described by K. A. Forbes et al., Comparison of Microwave-Induced Plasma Sources, J. of Analytical Atomic Spectrometry, Vol. 6, pp. 57-71 (1991); J. P. Matousek, Microwave-Induced Plasmas: Implementation and Application, Prog. Analyt. Atom. Spectrosc., Vol. 7, pp. 275-314 (1984); S. R. Goode et al., A Review of Instrumentation Used to Generate Microwave-Induced Plasmas, Applied Spectrosc., Vol. 38, pp. 755-763 (1984); and Zander et al., Microwave-Supported Discharges, Applied Spectrosc., Vol. 35, pp. 357-371 (1981).
It would therefore be desirable to provide methods and apparatus for analyzing solid, liquid and gas samples utilizing a self-powered portable, sensitive trace element sensor device. It would also be desirable to provide a compact, portable sensor device which is capable of characterizing samples containing more than one trace element simultaneously, thereby overcoming the shortcomings associated with the prior art.